Great efforts have been dedicated to the preparation of pharmaceuticals having minimum amounts of impurities. The control of impurities is a key parameter to evaluate the efficiency of a process and a huge number of variables have to be taken into account in order to select the reaction conditions and the control protocols necessary to ensure that marketed medicaments are pure and therefore safe.
The guidelines of the regulatory organizations, for example the Food and Drug Administration (FDA) of the United States, suggest that any impurities in the medicaments be identified, when present in amounts above 0.1% (i.e. 1.000 ppm).
It should be noticed that ppm means parts per million, therefore 1% corresponds to 10,000 ppm; 0.1 corresponds to 1,000 ppm; 0.01 corresponds to 100 ppm and 0.001 corresponds to 10 ppm.
Fesoterodine, namely (R)-2-(3-diisopropylamino-1-phenylpropyl)-4-(hydroxymethyl)-phenol isobutyrate, of formula (I)

is a compound with antimuscarinic activity, used in clinics in the form of the fumarate salt for the treatment of the overactive bladder syndrome and in particular of urinary incontinence.
U.S. Pat. No. 6,713,464 discloses the preparation of Fesoterodine through different synthetic methods, inter alia according to the Scheme reported below.
wherein P is a protecting group and R is hydrogen or alkyl.
Some of the known methods use as key intermediate a compound of formula (A), wherein R is hydrogen or methyl and P is a protective group for the phenolic hydroxyl, for example benzyl. Reduction with lithium aluminium hydride (LiAlH4) of the carboxylic function of a compound of formula (A) provides a compound of formula (B), which can optionally be protected at the phenolic hydroxyl, and, if the case after deprotection, by selective esterification affords Fesoterodine of formula (I).
However, it should be noticed that the acylation reaction of the phenolic hydroxyl of a compound of formula (II) when carried out according to known techniques such as in U.S. Pat. No. 6,713,464, is not sufficiently regioselective, due to the presence of the primary hydroxyl group in the intermediate of formula (II).

The low regioselectivity of the acylation reaction involves the formation of a compound of formula (III), a Fesoterodine regioisomer,

and of the diacylated compound of formula (IV)

These compounds and the process for the preparation thereof are known from U.S. Pat. No. 6,713,464.
The Applicant, using the acylation conditions already known, detected a further unknown impurity in the reaction crude, whose structure was found to correspond to compound of formula (V) by HPLC-MS and NMR.

This impurity proved particularly difficult to separate from Fesoterodine, even after conversion thereof to a crystallizable salt.
There is therefore the need for an alternative regio- and chemoselective esterification method which provides Fesoterodine or a pharmaceutically acceptable salt thereof, endowed with high regio- and chemoselectivity, and a low content in the impurities of formula (III) and/or (IV) and/or (V).